1. Field of the Invention
The present invention relates to a DC/DC converter and a subscriber line interface circuit (SLIC) using such. The SLIC is carried by a gateway unit such as a Voice over Internet Protocol (VoIP) gateway unit.
2. Description of Related Art
A gateway unit is a device for mutual connection of networks or systems using different protocols. In other words, the gateway unit conducts mutual conversion of different protocols. A VoIP gateway unit is known as one of gateway units. The VoIP gateway unit is a device for connecting a telephone network with the internet. In other words, the VoIP gateway unit conducts mutual conversion of a voice communication protocol and an internet protocol.
The VoIP gateway unit carries multiple SLIC. One SLIC accommodates one subscriber line. The SLIC executes a voice communication protocol between the telephones connected to the subscribed line, i.e. subscriber telephones. Chips of a large number of types have already been marketed as LSI (Large Scale Integrated circuits) for SLIC. For example, ProSLIC Si3210 (trade name) by Silicon Laboratory Inc. is known as a SLIC chip. With the SLIC chip, the values such as a voice level or ringer frequency can be set by a program. Those set values are stored in a register provided inside the SLIC chip. A SLIC module is composed by the SLIC chip and peripheral circuits.
A SLIC supplies a voltage to a subscriber telephone via a subscriber line. The voltage supplied to the subscriber telephone varies according to a communication state. For example, a SLIC supplies to a subscriber telephone a voltage −64 V in a waiting mode, −91.5 V when a ringer rings, and −15 V in an off-hook mode. Switching of the supplied voltage is controlled by the above-described SLIC. A SLIC comprises as one of peripheral circuits a DC/DC converter for converting a drive voltage (for example 9 V) into those voltages.
Abnormal heat generation in the DC/DC converter causes malfunction of the SLIC. Therefore, in order to ensure reliability of the SLIC, it is desirable that the temperature of the DC/DC converter be stringently controlled.